Flexible vinyl sheet, film, and flexible vinyl coated fabric are very versatile materials and find a wide variety of uses. Curtains for windows and showers, coverings for home and office furniture, automotive seat covers, single ply flexible vinyl membranes for covering industrial roofs, wall coverings, shoe uppers, purses and agricultural sheet illustrate the diversity of uses for this material.
Despite its wide utility there are some problems associated with the use of flexible vinyl in many applications. One of the more serious problems involves cracks or tears that develop in a flexible vinyl product after a period of use as the vinyl becomes stiff and brittle with age. These cracks or tears generally mark the end of the useful life of the vinyl.
It is known that loss of plasticizer from flexible vinyl is responsible for this stiffening and embrittlement of the vinyl (P. Dunn, et. at. J. Appl. Poly. Sci., 14, 2107-2116, 1970). The vinyl gradually loses flex fatigue resistance, becomes progressively shiffer, harder, and more brittle with further losses of plasticizer until cracks already develop.
Flexible vinyl film, sheet, coated fabric etc. can lose plasticizer by a variety of mechanisms:
(1) the most common cause of plasticizer loss and subsequent embrittlement of flexible vinyl materials is plasticizer volatilization.
(2) plasticizer loss by rub off represents the second most frequent cause of plasticizer loss.
(3) a less often encountered cause of plasticizer loss is extraction by cooking fats and oils which condense on flexible vinyl coated products located in the food preparation area of the home during cooking.
The number of methods of preventing plasticizer loss that have been proposed reflect the substantial economic implications of the problem. For example, it has been proposed to plasticize polyvinyl chloride with materials having molecular weights that are sufficiently high that plasticizer loss due to volatilization would be negligible. Such high molecular weight non volatile materials are generally not as efficient plasticizers for polyvinyl chloride resin as the low molecular weight more volatile plasticizers. Impermeable coatings for the surface of flexible vinyl film, sheet, and coated fabric have been proposed that would prevent loss by inhibiting plasticizer migration to the surface and subsequent evaporation from the surface of the flexible vinyl product.
Although all of the foregoing techniques for preventing plasticizer loss are effective to some degree they are not universally employed by manufacturers in the highly cost competitive flexible vinyl industry owing to their expense. All of the foregoing methods for preventing crack or tear development in flexible vinyls owing to plasticizer loss are designed to be implemented at the time the flexible vinyl product is being manufactured. For example, it is well known that plasticizers can be incorporated in vinyl plastic at the time when the vinyl plastic compound is being manufactured. Conditions which must be met in order to incorporate vinyl plasticizers in vinyl products at the time of manufacture according to the Plastics Engineering Handbook include temperatures of 300.degree. to 350.degree. F. in order to fuse the vinyl dispersion, high shear, low speed mixing in order to disperse the resin in the plasticizer, and the use of special very fine particle size, high porosity resins known as dispersion grade resins in order to insure that the plasticizer permeates the resin thoroughly.
A need, however, exists for a method of preventing plasticizer loss induced cracking after vinyl products have been manufactured, fabricated, and purchased by the consumer. U.S. Pat. No. 2,622,038 attempts to respond to this need by disclosing a method for preventing deterioration of vinyl plastics by applying plasticizer to the surface of the flexible vinyl plastic and allowing it to be absorbed. Although this method has proven useful it is not without its disadvantages.
When certain U.S. Pat. No. 2,622,038 plasticizers are applied neat to a flexible vinyl surface they do not wet that surface. Failure to wet the surface of vinyl products by these plasticizers leads to blisters, blotchiness, pigment extraction, and surface disfigurements on the surface of the vinyl product once the isolated pools of plasticizer on the product surface have been absorbed. Other U.S. Pat. No. 2,622,038 plasticizers when applied neat require excessively long times for absorption into the surface of a flexible vinyl product. In addition all plasticizers of U.S. Pat. No. 2,622,038 when tested are readily extracted from flexible vinyl products by kerosene, a test which assesses the sensitivity of flexible vinyl plastics to accidental spillage of automotive fluids. The number of plasticizers which are truly satisfactory for replasticization of flexible vinyl plastic products is remarkably small given the large number of plasticizers that are available commercially at this time.
When any plasticizer including U.S. Pat. No. 2,622,038 plasticizers are applied as a solution or dispersion in a diluent which will not attack flexible vinyl plastics such as mineral oil the surface of the vinyl plastic is wetted, but the rate of absorption of the solution or dispersion into the surface of the vinyl plastic is greatly retarded.